In recent years there has been an increasing demand for zoom lens systems that are provided in electronic imaging apparatuses, such as digital cameras, etc., to be more compact (miniaturized) and to have higher optical performance. Furthermore, there has been a demand to reduce the size (miniaturize) and weight of the focusing lens group to carry out a rapid and quiet focusing operation. Furthermore, there is also an demand to attain a superior optical quality by favorably correcting various aberrations such as axis chromatic aberration and lateral chromatic aberration, etc.
It is typical for a telephoto zoom lens system used in a compact digital camera to have a large f-number at the long focal length extremity due to miniaturization. However, since the aperture is small, the resolving power of the telephoto zoom lens system cannot be improved even if aberrations are favorably corrected due to the influence of diffraction. Accordingly, a zoom lens system in which aberrations are favorably corrected while having a smaller f-number is demanded; and developments have been made for meeting such demands.
For example, Patent Literature Nos. 1 through 3 each disclose a zoom lens system, having four lens groups, configured of a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in that order from the object side, wherein the first lens group is divided into a front sub-lens group and a rear sub-lens group, and the rear sub-lens group of the first lens group serves as a focusing lens group that is moved during focusing.
However, since the zoom lens system of each of Patent Literature Nos. 1 through 3 uses an extremely large number of lens elements in order to achieve a specified optical quality, the miniaturization and reduction in weight of the zoom lens system cannot be said to be sufficient. In particular, since the number or the specific gravity of lens elements that configure the rear sub-lens group (focusing lens group) of the first lens group is large, the miniaturization and reduction in weight of the focusing lens group is insufficient, so that a rapid and quiet focusing operation cannot be carried out. Furthermore, since the Abbe number setting of the lens elements that configure the front sub-lens group and the rear sub-lens group of the first lens group is unsuitable, it becomes difficult to correct various aberrations such as axial chromatic aberration and lateral chromatic aberration, thereby deteriorating the optical quality.